1. Field of the Invention
The invention relates to cutting elements and in particular to cutting elements for rotary drill bits, such as drag-type drill bits and rolling cutter drill bits. However, the invention may also be applicable to the manufacture of cutting elements for use in machine tools and the like.
2. Description of the Related Art
As is well known, one common form of cutting element for a rotary drag-type drill bit is a two-layer or multi-layer cutting element where a facing table of polycrystalline diamond is integrally bonded to a substrate of less hard material, such as tungsten carbide. The cutting element is usually in the form of a tablet, usually circular or part-circular. The substrate of the cutting element may be brazed to a carrier, usually also of cemented tungsten carbide, which is received in a socket in the bit body, or the substrate itself may be of sufficient axial length to be mounted directly in a socket in the bit body.
As is well know, polycrystalline diamond is formed by sintering diamond powder with a suitable binder-catalyst in a high pressure, high temperature press. Hitherto, the polycrystalline diamond employed in cutting elements for rotary drill bits has been of three basic types.
In the most common type, which will hereinafter be referred to as "conventional" polycrystalline diamond, the binder-catalyst is cobalt. In one common process for manufacturing two-layer cutting elements, diamond powder is applied to the surface of a preformed tungsten carbide substrate incorporating cobalt. The assembly is then subjected to very high temperature and pressure in a press. During this process cobalt migrates from the substrate into the diamond layer and acts as a binder-catalyst, causing the diamond particles to bond to one another with diamond-to-diamond bonding, and also causing the diamond layer to bond to the substrate.
Although cobalt is most commonly used as the binder-catalyst, any iron group element, such as cobalt, nickel or iron, or alloys thereof, may be employed.
The disadvantage with such conventional polycrystalline diamond is that the material is not thermally stable beyond about 750.degree. C., due to the presence of the metallic binder, which causes the diamond to graphitize. Also, the difference in coefficient of thermal expansion of the diamond and cobalt may also cause deterioration of the diamond layer with increase in temperature above about 500.degree. C.
In order to overcome these problems, so-called "thermally stable" polycrystalline diamond components have been produced and are sometimes used in drag-type drill bits. In one type of thermally stable diamond the cobalt or other binder-catalyst in conventional polycrystalline diamond is leached out of the diamond after formation. While this may increase the heat-resistance of the diamond to about 1200.degree. C., the leaching process also removes the cemented carbide substrate which leads to severe difficulties in mounting such material on a drill bit.
In an alternative form of thermally stable diamond, silicon carbide is used as the binder-catalyst. Again, the thermal resistance of the diamond is improved, but again difficulties are encountered in mounting the material for use on a drag-type drill bit.
More recently, a further type of polycrystalline diamond has become available in which carbonates, such as powdery carbonates of Mg, Ca, Sr, and Ba, or two or more types of these carbonates, are used as the binder-catalyst when sintering the diamond powder. Polycrystalline diamond materials of this kind are described, for example, in Japanese Patent Laid-Open Publications Nos. 74766/1992 and 114966/1992, the contents of which are incorporated herein by reference.
Polycrystalline diamond of this type has significantly greater wear-resistance and hardness than the types of polycrystalline diamond hitherto used as cutting elements in drill bits. The material is difficult to produce on a commercial scale since much higher temperatures and pressures are required for sintering than is the case with conventional and thermally stable polycrystalline diamond. One result of this is that the bodies of polycrystalline diamond produced by this method are smaller than conventional polycrystalline diamond elements. This, together with other characteristics of the material makes it difficult to mount bodies of the material in such a way that they may be used as cutting elements in rotary drill bits.
The present invention sets out to overcome these problems and to provide novel arrangements and methods for mounting polycrystalline diamond of this kind in a manner where the material may be used in cutting elements for rotary drill bits.